Rotating beacon antenna with strip line modulators



March 9, 1965 D. F. BOWMAN 3,173,142

ROTATING BEACON ANTENNA WITH STRIP LINE MODULATORS 5 Sheets-Sheet 1 Filed April 29, 1959 N M IWM March 9, 1965 D. F. BOWMAN ROTATING BEACON ANTENNA WITH STRIP LINE MODULATORS 5 Sheets-Sheet 2 Filed April 29, 1959 INVENTOR.

March 9, 1965 D. F. BOWMAN 3,173,142

ROTATING BEACON ANTENNA WITH STRIP LINE MODULATORS Filed April 29, 1959 5 Sheets-Sheet a INVENTOR. .0/9 0/0 F Bad/M4 March 9, 1965 n. F. BOWMAN 3,173,142

ROTATING BEACON ANTENNA WITH STRIP LINE MODULATORS ll U 1] l IV ll 1- n H 'n- 3 v /V A? 88 Fl n INVENT OR. 0141/10 F. Jaw/HAN BY & g %4 United States Patent 3,173,142 ROTATING BEACON ANTENNA WITH STRIP LINE MODULATORS David F. Bowman, Wayne, Pa., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Apr. 29, 1959, Ser. No. 809,690 13 Claims. (Cl. 343-754) This invention relates generally to microwave antennae, and more particularly relates to novel modulators for transmitter antennae.

While the present invention is generally applicable in the microwave art, it is particularly useful in the basic modulation of directional transmitters for Tacan and Vortac installations. Reference is made to my copending application Serial No. 742,646, now Patent No. 2,990,- 545, filed on June 17, 1958, for Broad-Band Antenna, and assigned to the assignee of this case. Each Tacan antenna transmits at a selected frequency in the low (60 to 1024 me.), or high (1150 to 1215 me.) Tacan bands. The Tacan radiation pattern has one component that is omnidirectional laterally, a second component that provides nine cycles of modulation, and a third component that provides a fundamental modulation. This pattern is rotated at 900 rpm. to provide corresponding 135 and 15 cycle signals.

The present invention is a continuation-in-part of the aforesaid application, with particular improvement in the means for modulating the lateral toroidal radiation pattern from that of the generally spherical Luneberg type sphere thereof. The resultant field distribution has the form:

where: is the angle in azimuth along the lateral pattern (from a starting reference point); and: (b :-b and (b +b are the modulation coefficients of the respective fundamental and nine-lobed modulations, ranging in the order of 0.1 to 0.3 as selected.

In accordance with the present invention I provide a radial transmission line capable of supporting a TEM mode between parallel discs or coaxial cones, with strip lines as radial transmission paths therein arranged to introduce controlled coupling between the strip transmission modes and the original TEM mode. The desired resultant variation in the field, is effected at or near the output radius of the strip line modulators as will be set forth in detail hereinafter. Also, an important feature of the invention hereof is to provide means to obtain a constant relative phase difference of 180 between the outputs of two adjacent strip lines by reversing the sense of coupling (between strip line and TEM modes) of one line with respect to the other.

The modulator system hereof has been found to provide materially improved control as to (a) constancy or selectively of frequency action in designed ranges; (b) purity of resultant waveform of modulations; (c) amplitude relationship uniformity, particularly for the nine-lobe modulations; and (d) constancy of input impedance of the antenna structure over the frequency range.

It is accordingly a primary object of the present invention to provide a novel modulator for an otherwise laterally omnidirectional microwave antennae.

Another object of the present invention is to provide a novel microwave modulator embodying strip line paths.

A further object of the present invention is to provide a novel microwave modulator combining a radial transmission TEM propagation mode with strip transmission modes.

Still another object of the present invention is to provide a novel microwave laterally omnidirectional transmitter embodying a plurality of radial strip lines arranged to modulate the basic field distribution.

Still a further object of the present invention is to provide a novel Tacan transmitter antenna with a central rotating modulator having a strip line array to effect multi-lobed azimuthal modulation in the radiated field.

These and further objects of the present invention will become more apparent in the following description of exemplary embodiments thereof.

FIGURE 1 is a vertical cross-sectional view through a Tacan antenna structure incorporating an exemplary strip line modulator array centrally thereof.

FIGURE 2 is a plan view of the central modulator as seen along the line 22 of FIGURE 1, in the direction of the arrows.

FIGURES 3, 4 and 5 are radial cross-sectional views through the modulator of FIGURE 2, taken respectively along the lines 33, 4-4 and 5-5 thereof.

FIGURE 6 is a transverse cross-sectional view through a modulator element, taken along the line 6-6 of FIG- URE 5.

FIGURES 7 to 10 are electrical diagrams used in describing the operation of the strip line modulators hereof.

FIGURE 11 is a vertical section through a modified form of modulator array.

FIGURE 12 illustrates a composite fundamental and multi-cycle modulator embodiment.

FIGURE 13 is a plan view through the modulator of FIGURE 12, as taken along the line 1313 thereof.

FIGURE 14 is a further modified form of the invention modulator.

The overall Tacan antenna embodiment with which the subject modulators coact is illustrated in FIGURE 1. The antenna structure 20 is comparable in arrangement and operation to that described and illustrated in my patent application referred to hereinabove, and reference is made thereto for detailed information. Briefly, the antenna 20 comprises upper and lower dielectric lens assemblies 21, 22 with a rotatable modulator 23 centrally therebetween. The modulator 23 is coaxially fed by rotating shaft 24 and concentric tube 25. Bearing 24a supports tube 25 in vertical rotation. Shaft 24 is rotated by motor 26. The basic microwave signal, at a predetermined carrier frequency for a given antenna structure, is fed to rotatable coaxial lines 24, 25 through rotary joint 27 fed through a flange 28 connection. A monitor antenna 29 is provided.

The antenna system 20 is supported on a frame base 30. An optical alignment device 31 is used, in conjunction with an adjustable alignment pad 32. Also a pulser wheel 33 with a pulser coil 34 are used for the Tacan signal system not pertinent to the present invention, as is tachometer-generator 35. An access door 36 is provided in base 30 with terminal boards 37 thereat.

The upper and lower lens assemblies 21, 22 constitute a modified Luneberg lens. A Luneberg lens gives essentially a pencil beam when illuminated by a point source. Antenna 20 requires a beam that is narrow in azimuth but Wide in elevation. This is done by modifying the spherical design of Luneberg to introduce curvature into the vertical plane wavefront. This may be effected by changing the external shape and internal surfaces of constant dielectric constant of the lower half 22 of the otherwise spherical lens to oblate spherical surfaces constituting the zones indicated at 22a through 226. The upper half section 21 is preferably shaped hemispherically as indicated with concentric zones 21a through 21c.

The conventional Luneberg lens is a spherically symmetric lens in which the dielectric constant varies with radius in such a manner that any point on the focal surface is transformed into a plane wave front.

It would be possible, but impractical, to construct the required lens of plain dielectric material because of material availability, weight and cost considerations. It is practical to achieve the required dielectric characteristics with an artificial dielectric composed of a dielectric matrix having a relatively low constant within which are spaced metallic inclusions 38 to increase the effective dielectric constant. The spacing density of the inclusions 38 is varied to effect the required change in dielectric constant from a value of 2 at the center to a value approaching 1 at the outside. This variation is done in discrete steps. Zones a halfwave thick constitute a good approximation herein.

The several zones 21a through 21e and 22a through 22e of lenses 21 and 22 are practicably constructed with dielectric layers or slabs. Such arrangement is illustrated diagrammatically at zone 22a with suitable dielectric layers to outline the zone form in three dimensions. The metallic inclusions 38 are preferably in the form of thin-wall, tubular inserts approximately 1 inch high. These inserts 38 are arrayed, as only partially indicated schematically, with their axes every where vertical in spite of the fact that the artificial dielectric thus formed is anisotropic. The effect of the anisotropy is small in the outer shells of the lens because little or no loading is required. The effect of anisotropy is small in the high-dielectric cores of the lens sections 21, 22 because there is little or no vertical component of the rays passing through these cores.

The transmitter energy is introduced to the composite lens 21, 22 through a radial transmission line and an annular metallic horn radiator 41. line is formed by a dielectric filled space 42 between two parallel metal discs 43, 44. This space 42 also contains the modulating system 45, 46 to be described in detail hereinafter. At the outer edge of the radial transmission line the energy is made to reverse direction and to travel toward the lens 21, 22 in two portions of the annular horn 41, one above and one below the radial transmission line 43, 44. The length and taper of the horn 41 (in the vertical plane) can be controlled to ensure that the energy is transferred effectively into the lens 21, 22. This step is important for if any considerable amount of energy is allowed to radiate directly from the horn 41 Without going through the lens, it will affect the secondary modulation adversely.

Such secondary modulation would be affected adversely by directly radiated energy because:

(1) The modulation phase of the direct radiation is 180 from being correct.

(2) The effective modulation coefficient of the direct radiation is widely variable with frequency and elevation angle since it is governed by the rules of the phasemodulator type of antenna.

(3) The direct radiation can form high-gain annular lobes directed at high elevation angles.

The energy division between the upper and lower portions of the horn 41 may be controlled by dimensional proportioning of the junction between the radial transmission line 43, 44 and the throat of the horn 41. In addition, the phase of the radiation from one portion may be delayed with respect to that of the other by means of dielectric loading, for example. By these means the secondary radiation components from the upper and lower portions 21, 22 of the lens may be made to cooperate most effectively to meet the vertical plane pattern requirements.

As referred to hereinabove, the function of the primary modulation system 23 is to form a circumferential variation of voltage of the form of formula bo+b1 sin Sin The radial transmission Cit where and b the primary modulation coefficients, are selected to be slightly larger than the desired corresponding secondary coefficients at the horizon, and where 4) is the angle measured around the circumference of the feed circle. The primary modulator system is rotated at 900 rpm. The top end 24 of shaft 24 connects with the center of plate 43 and constitutes a terminal to radial discs 43, 44 for microwave energy input. Sleeve 25 has its upper rim 25 connected to lower disc 44 to complete the coaxial RF coupling. Rotation of motor 26 rotates as a unit: shaft 24, tube 25, plates 43, 44, and modulation system 45, 46.

Stationary annular plates 47, 48 electrically couple with rotatable modulator system 23 through suitable resonant chokes indicated at 49, 49. Resonant chokes 49, 49 are formed of high and low impedance quarterwave sections. They allow rotation of the entire modulating assembly 23 while permitting uninterrupted radial flow of the energy, sinusoidally modulated with 9 cycles in 360 degrees, into the 360 modulation stator 47, 48. The transmission line run between the input flange 28 and the modulator assembly 23 is an effective RF coaxial line 24, 25. A simple rotary joint 27 is provided of such a configuration that the transmission line 24, 25 does not pass through the center of the motor shaft. The avoidance of a hollow-shaft motor 26 has decided advantages not only in the use of a more conventional motor but also in the much greater accessibility provided for motor servicing and replacement. The inner conductor 24 transmits the drive torque to the modulator assembly 23. To improve its torsional stiffness and at the same time permit impedance matching control, it is made about l-inch in diameter, or somewhat thicker than a SO-Ohm inner conductor for 1 /8 inch coaxial RF line.

The lack of symmetry caused by right angle entry of the feed line 27, 28 presents no problem to proper operation of Tacan or Vor portions of a navigation Vortac facility. Due to the cut-off waveguide action, energy converted by such right angle turn to any mode except the symmetric TEM mode is attenuated at least by a factor of 1O- in the vertical run of coaxial line 24, 25. This residual amount is negligible. The external symmetry of the antenna may be preserved for VOR considerations by routing the input RF cable so as to emerge from the center of the base 30. The coaxial transmission line 24, 25 passes through an active portion of the lower lens assembly 22. This tends to modify the vertical plane characteristics of the antenna. A dummy transmission line 50 in the upper half 21 of the lens is oriented vertically and symmetrically to line 24, 25. This line 50 counterbalances and eliminates any characteristic change caused by line 24, 25 in the lens system. With the addition of simple grounding provisions, the dummy line 50 is made to serve also as a lightning conductor for the protection of the upper part 21 of the lens. A static discharge jumper 51 is located at base line 50 along a divider plate 52 and centrally related to plate 43. A counter plate 53 is provided below modulator 23. A removable lifting hook 54 is securable to the top of unit 20.

FIGURES 2 through 6 illustrate a specific embodiment for rotatable primary modulator assembly 23, used in a 15 and cycle modulated Tacan transmitter. FIG- URE 2 is a plan view of the multiple-lobe modulator 23. Nine pairs of alternately positioned modulator sections 45, 46 create the nine-lobed configuration in the microwave field radiated. The fundamental lobe for the 15 cycle modulation is created, as by using a bolt 55 near center of, and between, radial transmission discs 43, 44 in the manner of the aforesaid application. An alternate way is illustrated in FIGURES 12 and 13 herein, and later described. The coaxial drive and RF transmission line to assembly 23 is indicated at 24, 25, centrally thereof.

The modulator sections 45, 46 comprise a flat annular plate 60 mounted parallel to plates 43, 44 and nearer one of them, such as to plate 44 in this case, and a plurality of equi-spaced radial strip lines 63. Annular plate 60 in the preferred example, is proportioned to be M2 long in the radial direction, where A is the wave length of the applied microwave energy. Annular plate 60 is mounted to disc 44 along its radial center locus 61, namely at its M 4 position (see FIGURE 3) by a series of closely spaced bolts 62. Also, a series of end bolts 62a can be used to secure plates 60 and 44 as merely indicated in FIG- URE 2. One for each modulator grouping (45, 46) may be used.

As will be set forth, the central locus 61 is the signal nodal position, effected by the circular array (or ring) of bolts 62 between annular plate 60 and disc 44. Spaced bolts 62, 62 are accordingly used for this purpose. As an alternate, an arcuate plate 62b is indicated in FIG- URE 2. A plurality of strip line inner conductors 63, namely flat metallic bars (or narrow sectors) are supported centrally between annular plate 60 and disc 44. The strip lines 63 are at equally spaced radial positions. There are eighteen such inner conductors 63 to effect the nine lobed modulation hereof, to be described in more detail. The lines 63 are spaced 20 apart herein. Other arrays and number of strip lines may be used. Each strip line 63 is coextensive with the annular plate 60, and oriented radially. Strip lines 63 are accordingly also M2 in length. They are spaced between the bolt 62 array and not connected thereto. Each end of the strip lines 63 are secured to, and electrically connected with either annular plate 60 or the disc 44, in a manner to be set forth in more detail.

The phase reversing or negative modulator sections (45), see FIGURES 3 and 5, have their radially inner strip ends 63a connected to lower disc 44 by a metallic post 64; and their outer ends 63b to annular disc 60 by a metallic post 65. Posts 64, 65 are narrow radially, and extend transversely to the extent of the Width of strip line ends 63a and 63b. The positive or non-reversing modulator sections 46, see FIGURE 4, have both end-shorting posts 66, 67 connected to a common plate, e.g. 60 or 44. In FIGURE 4 strip line radially inner end 63a has a post 66 connected to disc 44; and outer end 63b, with post 67 thereto.

The alternated arrangement along rim of modulator assembly 23 of positive and negative modulator sections 46 and 45 respectively, provides the multilobed pattern, nine lobes or cycles in this example. The annular plate 60 and strip lines 63 are also ruggedly sup ported in dielectric material 68 between main discs 43, 44, as illustrated. Material 68 may be of low-loss dielectric foam, polyfoam, Styrofoam, etc. It is understood that all the integrated elements of the modulator assembly 23, as discs 43, 44, sector plates 60, bolts 62, strip lines 63 and the associated posts are of conductive material. Their electrical function and action is now described.

FIGURE 7 is a diagnamrnatic showing of a modified modulator section 45 corresponding to phase-reversing sections 45 (FIGURES 3 and 5). The nodal points (61') are derived by shorting bolts 62 in circumferential array (as per FIGURE 2), between outer plates 43', 44'. In this section 45' the basic differences are that the annular plate 60 is integrated with the basic upper radial transmission line disc 43, and that the discs 43, 44 extend beyond strip line 63. The basic operation of the modulator sections 45' per se is the same as that of sec tions 45 of FIGURES l to 6. However, in modulator 45' (of FIGURE 7) the full impressed wave a is modulated and phase-reversed by 180 at output b. This corresponds to b =0 in Formula 1, with all energy converted to b cos n.; which in a nine lobed pattern is 11,, cos 941.

In modulator section 45 (see FIGURE the modulation factor corresponds to the relative position of annular plate 60 between discs 43, 44. An unmodulated component is transmitted radially above modulator section 45, i.e. between plate 60 and disc 43. The phasereversed output component of modulator 45 combines with the unmodulated component passed above section 45 to constitute modulation thereof at the output. Suitable adjustment of the distance between plate 60' and disc 44 provides the desired modulation ratio or factor (b +b The other differences between modulation sections'45 and 45' (i.e. FIGURE 5 vs. FIGURE 7) is the radial extent of discs 43, 44 with respect to that of strip lines 63. This indicated in FIGURE 7 by r as the radial extent of strip line 63; r that of disc 43 (and of 44'). In constructing a modulator assembly (23) we use the following relation:

where:

n=number of full cycles of modulation, v=wave length of basic signal, r =radial extent of each strip line (63).

It is clear that if 21rr is less than M, then it is not feasible to obtain n uniform modulations about the azimuth. With r greater than r a filter effect is provided with a cut-off above the n frequency modulations. This keeps the antenna radiations pure of harmonic output. The modulator sections 45 and 45' work equally for r equal to or greater than r For Vortac installations, per assembly -20, the r =r version is preferred. For mobile Tacan use, r greater than r is preferred.

Referring to FIGURE 7, the modulator section (45') is situated in the radial tnansmission line (43', 44') at a radial position (a) wherein the inner end 63a of strip line 63 is at a peak region of a standing wave between the discs (43, 44). The shorting bolts 62 cause a shorting ring due to their closeness, resulting in zero voltage .thereat between plates 43 and 44 in FIGURE 7 and annular plate 60 and plate 44 in FIGURES 3 and 5. This zero signal or nod-a1 radial point is indicated at (c) in FIGURE 7. A standing wave of sinusoidal shape is thereby produced by the bolt 62 ring. The peak region (a) of this standing wave is coupled to strip 63 at strip end 63a at the indicated point (a) along the signal strength distribution curve 70 for modulator section 45'. FIGURE 8a illustrates the transverse electromagnetic fields (TEM), transverse .to the direction of propagation at position (a) in unit 45.

The end 63a of strip 63 is at low inductance to plate 44' due to the shorting post 64 interconnection. The T EM fields etxend from strip end 63a to upper plate 43 at (i) in the same direction or phase as the initial impressed field (ii). The strip line 63 being grounded at initial point (a), distorts the true standing wave caused by bolt 62 ring, and couples the wave outwardly radially along the strip 63.

As the coupled field is propagated radially outwardly, the typical field distribution at region (b) is denoted in FIGURE 8b. The strip mode has TEM fields to both plate 43' at (iii) and plate 44' at (iv). The direct propagation at (v) is in original phase, and of reduced magnitude in the standing wave pattern. At the nodal region (61) the original and net field is reduced to zero, and upper and lower strip mode fields balance out as indicated in FIGURE 8c. The shorting bolt 62 ring is indicated thereat. Further along, at position (d), there is a reversed phase field (vi) evident in the steady state, with the strip mode above and below strip 63 indicated at FIGURE 8d. Energy coupled at (a) into the strip 63 passes shorting bolts 62, whereas the original field cannot due to shorting ring 62. However, in the steady state condition resulting at end position 8e, the standing wave is established in reverse back to (d) and (c) for field (vi). At position (e) the strip end 63b is shorted by post 65 to upper plate 43', and the TEM field exists between the strip 63 and lower disc 44' at (vii). The field (viii) between plates 43', 44' is full and reversed from that of (ii) at (a). Curve 70 is sinusoidal, and onehalf wave length long at the steady state.

Thus it is seen that a strip line (63) with end shorting posts (64, 65) .to the opposite discs, causes a full phase reversal at output position (e) as signal b of the signal impressed at input a in a one-half wave length path. This is the action of phase-reversing modulator sections 45 and 45'. The alternate positive modulator sections 46 (or a corresponding one, not shown, where plates 60 are integrated with disc 43' per FIGURE 7), have strip end shorting bars (66, 67) connecting to either one or the other of discs 43, 44 at the same time. No phase reversal occurs and the amplitude output in phase with the main field passed between plates 60 and 43 (FIGURE 4) is additive.

FIGURE 9 is a simplified schematic diagram of the two types of modulator sections 45, 46 and their relation to the coaxial line (24, 25) input, the radial transmission line (43, 44), and the output. The parallel spaced discs 43, 44 may be replaced by spaced cones as described in the aforementioned applications. The inner basic fields pass from input a, a along transmission line 43, 44 to standing waves at a, a, the inputs to sections 45, 46. There are 2nd individual modulator sections (45, 46) alternated about the discs 43, 44 as shown in FIGURE 2.

The phase-reversing section 45 has energy split into two paths 71, 72. The phase reversal is indicated by transformer 73, 74 and transposed conductor 75, with paths 71 and 72 recombining in reversed phase sense. The output strength 19 is thus reduced from input Conversely, the input to section 46 split into two parallel paths 76, 77 and recombined in the same phase sense at ,3. The alternate outputs 6' and ,3 are fed into horn (41) or other suitable external radiator The positive modulation sections (46) produce crests of the desired modulated voltage distribution, while the negative sections (45) produce troughs. FIGURE illustrates the effective sinusoidal field distribution 80 about the peripheral output of discs 43, 44 due to the alternate modulator sections (45, 46). The alternate strip lines 63 serve an important function as exciters: they take a TEM wave, convert it to a strip line mode, conduct it through the nodal region (61), convert it back to TEM field in either the positive or negative sense, and modulate by combining with a parallel passed original field portion where so combined. In FIGURE 10 the basic alternate plus and minus crests are indicated across successive strip lines 63. There are no resultant square waves (81) or higher frequency modes. A pure continuous annular multi-lobed sinuisoidal modulation is effected of the basic propagated field, as will now be understood by those skilled in the art.

As these systems are in the microwave range and tailored to the wave-length fundamental (h), higher modes above the fundamental are filtered out, being cutoff and not propagated beyond the edges of discs 43, 44. The basic modulation of .the field transmitted by assembly (FIGURE 1) is effected by the lobes due to modulators 45, 46 (and the fundamental) together with the modulator assembly 23 being rotated. The modulated field output is radiated directly in space or through coupled stationary wave guides and radiators.

FIGURE 11 shows a further modified form of modulator system. This system comprises a full central disc 60:: with alternate positive and negative modulator sections 45", 46" between disc 60a and lower disc 44". The annular plate 60 is replaced here by the intermediate disc 600, as in FIGURE 7. A central aperture 85 in disc 60a passes TEM field from coaxial line 24, across the radial path between upper disc 43" and intermediate disc 60a, to the peripheral output (41). The output b is with reduced crest; output b', with enhanced crest.

The modulator sections may also be used to create the fundamental modulation namely b cos see Formula 1. FIGURES 12 and 13 illustrates such arrangement in conjunction with the multi-lobe modulation b cos n.. The upper circular plate 86 is connected to inner coaxial conductor 24, lower circular plate 87 to conductor 25' to constitute the radial transmission line for the modulator assembly. The phase-reversing multilobe modulator section 88 corresponds to previous sections 45, with alternate positive modulators (not shown).

Near the center of disc 86 is located the single lobe or fundamental modulator sections 90, 91. The phasereversing section 90 is long radially, with a plurality of bolts 93, 93 along the nodal region at M4. The lower plate 94 is about 180 in extent. The intermediate strip line 95 is trapezoidal in shape, with its ends shorted by posts 96, 97 to the respective plates 94 and 86. The positive phase modulator section 91 is similar in size and shape to section 90, in general mirror symmetry. However, the end shorting posts 98, 99 are connected only between strip line 100 and upper disc 86. The intermediate sector plate 101 is supported along the nodal region by bolts 102.

FIGURE 14 is a modified strip line form for a modulator section, shown as a phase-reversing arrangement. The strip line 105 has two end portions 106 and 107 that are in length, to effect the conversion-exciter action of the above described strip lines. The intermediate strip portion 108 may be of any desired practical length while maintaining the strip line action of the other modulator sections described. Strip line 105 is located between two radial transmission line plates 110, 111 fed by coaxial line 24, 25'. The plates 110, 111 are spaced by rings of bolts 114 and 115 along the nodal positions about the plate to form two shorting rings. The strip line 105 is supported between plates 110, 111 by two shorting end posts 116, 117 arranged oppositely to the plates and 111 to effect the phasereversing mode corresponding to modulator section 45 described.

With shorting end posts 116, 117 arranged to connect the ends of strip line 105 to one or the other plates 110, 111 the positive modulator mode is effected corresponding to sections 46 hereinabove. The length of intermediate strip line portion 108 does not change the strip line mode of propagation which converts the arrested TEM field back to TEM when passed bolts into portion 107 outwardly of the modulator.

While the present invention has been described in connection with specific embodiments and applications, it is to be understood that modifications and variations may be made therein that fall within the broader spirit and scope thereof, as defined in the appended claims.

I claim:

1. A rotatable modulator for a microwave transmitter antenna comprising a radial transmission line for generating an omnidirectional radiation pattern, a transmission cable for feeding microwave energy to said line, and means extending radially in said line for about one-half wave length for modulating the microwave field distribution therethrough including a strip line with its ends connected to said line.

2. A modulator as claimed in claim 1, further including second modulating means oriented adjacent to the first said modulating means and arranged in opposite phase field output relation therewith to establish a lateral modulation lobe in the field propagated by the antenna.

3. A modulator as claimed in claim 1, further including a plurality of modulating means arranged radially between said line with the first said modulating means in alternate phase output relation to establish a lateral propagation pattern with a plurality of sinusoidal modulation lobes in the field propagated by the antenna.

4. A modulator as claimed in claim 3, further including a fundamental modulation means arranged centrally of the radial transmission line to establish a combination fundamental and multi-lobed lateral propagation pattern for the line.

5. A rotatable modulator for a microwave transmitter antenna comprising two parallel spaced discs, a transmission cable for feeding microwave energy to said discs, and means extending between said discs for about one-half wave length for modulating the microwave field distribution therethrough at angularly separated regions to thereby form a multi-lobed radiation pattern; said means including a strip line.

6. A rotatable modulator for a microwave transmitter antenna comprising a radial transmission line provided with two parallel spaced plates, a transmission cable for feeding microwave energy to said plates, and means extending radially between said plates for modulating the microwave field distribution therethrough including a strip line with its ends connected to said plates, and an effective shorting ring established along both plates to create a nodal region therein.

7. A rotatable modulator of microwave field energy of the character described comprising a radial transmission line provided with two parallel spaced discs, a transmission cable for feeding microwave energy to said discs, and means extending radially between said discs for modulating the microwave field distribution therethrough including a strip line with its ends shorted to opposite ones of said discs thereby effecting a phase reversal of the energy.

8. A modulator as claimed in claim 7, in which said strip line is substantially one-half wavelength long.

9. A rotatable modulator for a microwave transmitter antenna comprising a radial transmission line provided with two parallel spaced discs, a transmission cable for feeding microwave energy to said discs, and means extending radially between said discs for modulating the microwave field distribution therethrough including a strip line with its ends connected to one of said discs, and an effective shorting ring along the central portion connected to both said discs.

10. A rotatable modulator of the character described comprising two parallel spaced discs, a transmission cable for feeding microwave energy to said discs, and means extending radially between said discs for modulating the microwave field distribution therethrough including a strip line and a plate mounted adjacent to said strip line for diverting microwave energy in said modulator towards said strip line, and connection means for said strip line to eifect microwave energy propagation through the strip line section and radially outwardly of the discs to form a multi-lobed radiation pattern.

11. A modulator as claimed in claim 10, in which said strip line and said plate are substantially one-half wavelength long.

12. A rotatable modulator for a microwave transmitter antenna comprising a radial transmission line provided with two spaced plates, a transmission cable for feeding microwave energy to said plates, and means extending radially between said plates for modulating the microwave field distribution therethrough including a strip line, a third plate mounted adjacent to said strip line, a shorting member connecting to the central portion of said third plate with the one of said two spaced plates adjacent the strip line to create a nodal region, and connection means for said strip line to effect a strip line mode of propagation radially outwardly across the modulating means.

13. A rotatable modulator of microwave field energy of the character described comprising a radial transmission line provided with two parallel spaced discs, a transmission cable for feeding microwave energy to said discs, and means extending radially between said discs for modulating the microwave field distribution therethrough including a radial strip line and an annular plate mounted parallel to and between said discs, a line conductor connecting one end of said strip line to said annular plate, a second conductor connecting the other end of said strip line to the adjacent one of said transmission discs, means for short circuiting the radially central portion of said annular plate to said one transmission disc whereby a strip line mode of propagation radially outwardly of the modulating means phase reversal of the energy therein is effected.

References Cited in the file of this patent UNITED STATES PATENTS 2,527,222 Iams Oct. 24, 1950 2,572,041 Litchford et al Oct. 23, 1951 2,677,766 Litchford May 4, 1954 2,895,134 Sichak July 14, 1959 2,939,141 Casabona et al May 31, 1960 2,978,702 Pakan Apr. 4, 1961 

7. A ROTATABLE MODULATOR OF MICROWAVE FIELD ENERGY OF THE CHARACTER DESCRIBED COMPRISING A RADIAL TRANSMISSION LINE PROVIDED WITH TWO PARALLEL SPACED DISCS, A TRANSMISSION CABLE FOR FEEDING MICROWAVE ENERGY TO SAID DISC, AND MEANS EXTENDING RADIALLY BETWEEN SAID DISC FOR MODULATING THE MICROWAVE FIELD DISTRIBUTION THERETHROUGH INCLUDING A STRIP LINE WITH ITS END SHORTED TO OPPOSITE ONES OF SAID DISC THEREBY EFFECTING A PHASE REVERSAL OF THE ENERGY. 